# Thermal performance

The thermal power is a physical quantity that indicates the thermal energy converted in a period of time in relation to this period of time. It is a characteristic parameter of an energy conversion system. It is usually specified in kilowatts (kW) or megawatts (MW) and describes the heat flow .

## Definition for steam boilers, heat generators and the like

In a steam power plant from the boiler generated steam contains energy. The total energy results from the amount of steam, the steam temperature and the steam pressure. In the case of a steam boiler , the thermal output corresponds to the heat content of the added fuel minus the losses through exhaust gas ( chimney ) and further heat transfer from the system to the environment ( radiation , heat conduction and convection , including through the ventilation inside the boiler house).

The efficiency of the steam generator (steam boiler) is calculated from the ratio of the thermal power to the chemical energy of the fuel . The system's own requirements are not taken into account here. Until the net energy output of a steam power plant in the form of electrical energy into the power grid must still be considered a series of losses, u. a. for the condensate heat (approx. 55% of the thermal output, unless part of it is used as district heating ) and the internal consumption (approx. 8% of the generator output ).

The thermal power is also used in the calculation of the theoretical total energy potential, for example in order to estimate the necessary cooling of a power plant or the possibility of using district heating. A complete use of the entire heat flow is not possible, however, since losses always occur in the entire transmission system (friction, leaks, etc.). Losses also occur when the electrical current is transmitted, so that the electrical output does not allow any conclusions to be drawn about the energy actually used by the end consumer. The number of households supplied, which is often given for power plants, is therefore only an estimate.

## Definition of combined heat and power

In another sense, thermal power is the term used to describe the release of heat from a combined heat and power plant, in which houses are heated and process heat is used for industry. This is intended to make sensible use of the waste heat , which in large power plants averages 62%. With this consideration, the output thermal power and the electrical power can be added together.

The following table shows the ratio of electrical power to waste heat that is released at the location of the power plant, i.e. without transmission losses to the end user.

 Machine, process Energy used Waste heat electrical power % Nuclear power plant nuclear 65-67 33-35 Combined cycle power plant ( natural gas ) chemically 40-50 50-60 Thermal power plant ( coal ) chemically 50-75 25-45 Power plant with combined heat and power chemically 18-40 60-82